1. Field of the Invention
The present invention relates to a thermal flow sensor that causes a heating resistor to generate heat to measure a rate of a fluid flowing around the heating resistor, and in particular, to a thermal flow sensor including a heating controller for controlling an amount of heat generated by a heating resistor.
2. Background Art
As airflow sensors that detect intake air flows of internal combustion engines in automobiles or the like, thermal airflow sensors that can directly measure mass flow rates are generally used.
In recent years, a thermal flow sensor has been proposed in which a sensor element of the thermal flow sensor is produced on a semiconductor substrate such as silicon (Si) using a MEMS (Micro Electro Mechanical Systems) technology. Such a semiconductor sensor element has a cavity portion created by cutting a part of a semiconductor substrate in a rectangular shape, and a heating resistor (hereinafter, referred to as the “heater”) is formed on a dielectric film of several microns thick on the cavity portion.
In such a sensor element, a temperature-sensitive resistor (hereinafter, referred to as the “detection resistor”) is formed near a heater, and a flow rate is detected on the basis of the amount of heat transferred from a fluid flowing over the heater to the detection resistor. Therefore, the temperature difference between the heater and the ambient temperature needs to be constant. As a way to control a heater temperature, an indirect heat control scheme is known in which a temperature-sensitive resistor for monitoring a heater temperature (hereinafter, referred to as the “indirect heat resistor”) is formed near a heater, and a heater driving power is feedback-controlled so that a resistance value of the indirect heat resistor has a desired value.
A heater, a detection resistor, and an indirect heat resistor such as those described above are small in size, for example, several dozen to several hundred microns, and since they are formed on a thin layer of several microns thick, they have small heat capacities and are quick in response. Thus, delay in feedback control response or delay in heat transfer between a heater and an indirect heat resistor may prevent heater temperature from being appropriately controlled at the time of startup. A conventional art to solve the problem has been described in JP Patent Application Publication No. 2009-097925 A.
A thermal flow sensor described in JP Patent Application Publication No. 2009-097925 A inputs voltage signals, which vary depending on a temperature of an indirect heat resistor, to an up-down counter via a comparator. Output of the up-down counter is converted into electric power via a digital/analog converter (hereinafter, the D/A converter) and a transistor for driving a heater, and supplied to the heater. In addition, at power-on, an initial value of the up-down counter is set at a reference value (preferably, a maximum output indication value of the D/A converter) and the up-down counter is operated with a faster clock signal, whereby the amount of heat generated by the heater can be quickly controlled.